Wikimedia Commons requires that movies be uploaded as Ogg-Theora (OGG) files. As of Blender 2.42a, this is not a builtin feature of Blender. To get OGG files from your finished animation isn't difficult, though. However, you'll need additional software.

There are basically two ways to generate OGG files: you can use one of the many fine video editors or you can use special conversion programs. Video editors like LiVES or Cinerella allow you to load your AVI or your rendered frames, manipulate them, and create the OGG file from it. Please refer to the editor's documentation on how to achieve this.

A disadvantage of a video editor is they are huge pieces of software, duplicating functionality that you already used when you created your animation file/s with blender. It's actually not necessary to install a video editor just for converting your animation to OGG Theora format.

It's actually possible to convert frame pictures that you saved before to OGG format movies. The ffmpeg2theora software package, which is available in source or binary for all relevant systems, is capable of batch-processing files into an Ogg format movie. For example, if your frames were saved as PNG (with filenames filename001.png, filename002.png, etc.), you could convert them to a soundless OGG file with:

ffmpeg2theora filename%03d.png -o output.ogv

Sound is possible too, as well as being able to set the quality and framerates. Consult the ffmpeg2theora documentation for more.